1. Technical Field
The present invention relates to a radiation analyzer including a radiation detector provided with a transition edge sensor and to a method for analyzing radiation.
2. Related Art
There are radiation analyzers capable of energy discrimination of radiation such as an energy dispersive spectroscopy (EDS) and a wavelength dispersive spectroscopy (WDS).
The EDS is an X-ray detector that converts the energy of X-rays captured therein to an electric signal and derives the energy depending on the intensity of the electric signal. The WDS is an X-ray detector that monochromatizes X-rays (energy discrimination) with a spectroscope and detects the monochromat zed X-rays with a proportional counter tube.
Semiconductor detectors such as a silicon-lithium (SiLi) based detector, a silicon drift detector, and a germanium detector are known as EDSs. The silicon silicon-lithium based detector and the silicon drift detector are frequently used in the electron microscope of an elemental analyzer and are capable of detecting energy in a wide range of 0 to about 20 keV. However, their energy resolution cannot be readily improved to 130 eV or more and remains merely one-tenth or less of that of WDSs. This is because these detectors contain silicon, and thus their characteristics depend on the bandgap of silicon (about 1.1 eV) in principle.
Here, the energy resolution, which is one of the indexes indicating the performance of an X-ray detector, is described. The energy resolution of 130 eV means that detection can be done with uncertainty of about 130 eV when the X-ray detector is irradiated with X-rays. That is, the smaller the uncertainty is, the higher the energy resolution is. For example, in the case of detecting characteristic X-rays having two adjacent spectral lines with a difference of two adjacent peaks of about 20 eV, energy resolution of 20 to 30 eV is sufficient to separate the two peaks in principle because the uncertainty is reduced as the energy resolution is increased.
In recent years, demands for superconductive X-ray detectors of energy dispersion type having energy resolution equivalent to that of WDSs have been growing. A transition edge sensor (TES) among superconductive X-ray detectors is a high-sensitive thermometer utilizing a steep change in resistance (ΔR≅0.1Ω at ΔT≅a few mK) of a metallic thin film appearing at a transition between the superconductive state and the normal conductive state. Note that a TES is also referred to as a microcalorimeter.
The TES is used to analyze a sample by controlling the change of temperature in the TES in response to incident fluorescent X-rays or characteristic X-rays emitted from the sample irradiated with primary X-rays or primary electron beams. The TES has a higher energy resolution than that of other detectors. For example, it can achieve energy resolution of 10 eV or less with the characteristic X-rays of 5.9 keV.
A TES provided to a scanning electron microscope that includes an electron source such as a tungsten filament can capture characteristic X-rays emitted from a sample irradiated with electron beams and thus easily separate peaks of the characteristic X-rays (Si-Kα, W-Mα, β) that are difficult to separate by a semiconductor-based X-ray detector.
In X-ray analyzers including such superconductive X-ray detectors, an amplifier based on a superconducting quantum interference device (SQUID) is used to detect a small current variation in the TES. It is significant to keep a current flowing in the SQUID amplifier to achieve higher energy resolution of the TES. As described later, it is needed to reduce current variations in a current flowing in the SQUID amplifier to a small degree to provide higher energy resolution.
An X-ray analyzer is known as an apparatus that keeps a SQUID amplifier current, or a baseline TES current, constant. This X-ray analyzer corrects a current flowing in a TES or the peak value of the current, for example, depending on the fluctuation range of a baseline current flowing in the TES from a specific value if the baseline current deviates from the specific value and fluctuates (refer to JP-A-2009-271016).
The above-mentioned X-ray analyzer, however, requires constant monitoring of a baseline current. In the case where a TES detects no X-ray, a SQUID amplifier outputs a constant baseline value continuously. When the TES detects an X-ray, a signal pulse is superimposed on a baseline. When the number of X-rays is small, a period between one pulse and the next one is sufficiently long, and thus a baseline can be monitored in the period between the signal pulses. By contrast, when the number of X-rays is large, a period between one pulse and the next one is short, that is, the period is not enough to detect a baseline, and thus it is hard to monitor accurate baselines. Thus, there is a problem in that accurate determination of a baseline is difficult when a large number of signal pulses are detected.